1. Field of the Invention
The invention pertains to a method for shifting an automatic multi-stage transmission of a motor vehicle, where, to engage and disengage gear stages of the transmission, shift couplings can be actuated by assigned actuators.
2. Description of the Related Art
Shifting operations of automatic multi-stage transmissions such as manually shifted automatic transmissions or dual-clutch transmissions are usually controlled and regulated by a control unit, which sends and receives signals to and from the engine, the clutch device, and the transmission. The clutch and the transmission are actuated by assigned actuators, which are commanded by the control unit. During a shifting operation from a starting gear to a target gear, the clutch is opened by the clutch actuator to disconnect the drive train. Then, by means of the actuator assigned to the transmission, the starting gear is disengaged and the new target gear is engaged. Next, the clutch is closed again by the actuator to reconnect the drive train again, so that torque can be transmitted from the engine via the clutch and the new gear stage to the drive wheels of the vehicle. When just starting off, a corresponding procedure is used, namely, upon recognition of the desire to start off, the clutch is opened or kept open by the clutch actuating system; the target gear is automatically engaged; and finally the clutch is closed again so that torque can be transmitted.
The sole FIGURE is based on FIG. 1 in EP 1 835 204, which discloses an example of a manually shifted automatic transmission. In the FIGURE, an internal combustion engine 2 drives the input 3 of a double clutch mechanism 4, either directly or via a torsional vibration damper (not shown). The double clutch mechanism 4 consists essentially of the two clutch arrangements 6, 8, which are designed as dry clutches. The clutch arrangement 6 is assigned here to the first transmission input shaft 10, and the clutch arrangement 8 is assigned to the second transmission input shaft 12 of the double clutch transmission. The two clutch arrangements 6, 8 can be operated independently of each other by an electromechanical clutch actuator 14, which is controlled by a control unit 16. The gear stages 1, 3, 5 are assigned to the inner input shaft, and the gear stages 2, 4, 6 are assigned to the outer input shaft 12. A reverse gear stage R is also shown in FIG. 1. By means of the transmission actuator 18, the loose wheels of the gear stages can be connected for rotation in common to the transmission input shafts 10, 12 by way of the synchronizing units 20 and/or to the transmission output shaft 22, as a result of which the gear stages are engaged and the transmission output shaft 22 is connected for rotation in common by the engaged gear stage to the transmission input shaft assigned to that gear stage. A selector lever 24, furthermore, which can be moved into the various selectable positions P, R, N, D; a foot-operated service brake 26; and an engine control unit 28 are also connected to the control unit 16. Each synchronizing unit 20 includes a shift coupling and a synchronizing device.
When the selector lever is moved into the parking lock position “P”, the control unit 16 or the engine control unit 28 determines whether the internal combustion engine 2 is running. By means of the actuator 18, furthermore, a gear stage of each input shaft 10, 12 is engaged while the clutch arrangements 6, 8 are open. If the engine 2 is running, it is turned off automatically by the control units 16, 28, and the two clutch arrangements 6, 8 are closed by the actuator 14. To release the parking lock, the service brake 26 must be actuated to allow the selector lever 24 to be moved. When the selector lever 24 is moved from the parking lock position “P” into one of the other positions “R”, “N”, or “D”, at least one of the clutch arrangements 6, 8 is opened by the actuator 14. So that the internal combustion engine can be started, the clutch input 3 must be disconnected from the transmission output shaft 22, which can be done by having the actuator 14 open the two clutch arrangements 6, 8 and/or by having the actuator 18 disengage the engaged gear stages. A hydraulic unit 15 controls the flow of cooling oil in the transmission.
In the prior art, various methods for controlling the shifting operations of manually shifted automatic transmissions have already been disclosed. Thus, for example, DE 199 28 374 C1 describes a method in which, while the vehicle is being driven, a neutral position is selected by means of a shift lever. It is true that, if the vehicle is being driven, the clutch will be opened automatically in this case, but the currently engaged gear will not be disengaged as long as the vehicle is still moving.
Another method for shifting an automatic transmission is known from DE 198 45 604 C1. Here a shifting coupling of the transmission is pretensioned toward the open position of the transmission shift coupling by the transmission actuator even before the friction clutch, which is opened and closed by a clutch actuator, is completely open. Additional methods for shifting automatic multi-stage transmissions are known from U.S. Pat. No. 6,951,526, for example. Here, among other things, a method for use at low temperatures is proposed, according to which, to engage a transmission gear, several synchronizing devices of different gears are pretensioned in the gear-engaging direction to synchronize the speed of the corresponding transmission input shaft.
Especially in the case of automatic multi-stage transmissions with wet-running clutches, the drag torque transmitted via the clutches as a result of the high viscosity of the cooling oil at low temperatures has proven to be a problem. Under these conditions, the cooling oil adheres between the driven engine-side plates and the nondriven transmission-side plates and thus transmits a drag torque from the driven to the nondriven plates. Unless one of the gears assigned to the transmission input shaft is engaged in the transmission, these drag torques cause the engine-side half of the clutch to carry the transmission input shaft along at a certain rotational speed, which can be as high as the speed of the engine. When the vehicle is stationary, these rpm's cause comfort problems when a gear is engaged, because the shift coupling of the gear to be engaged must transmit a synchronizing torque to brake the transmission input shaft to a halt. To engage a start-off gear under such conditions, either the transmission actuator must exert a larger amount of synchronizing force, or the synchronizing time increases beyond a level which is acceptable to the driver.
In the prior art, solutions according to which a gear remains engaged while the transmission selector lever is in the neutral position have already been proposed. In this position, only the clutch is opened or remains opened. The disadvantage here, however, is that a defect in the clutch control unit can cause the clutch to close, and the vehicle can start off even though the selector lever is in the neutral position, which is undesirable. It is true that this safety risk could be eliminated by providing the clutch with a redundant check valve, so that a simple defect of the clutch control unit cannot lead to a situation in which the vehicle drives off while in neutral as described above. As a result of this measure, however, both the control unit and the hydraulic system (in a vehicle with a hydraulically controlled clutch and transmission) become more complicated, larger in size, and more expensive.